1. Field of the Invention
The invention relates to an electronic device with a housing, a rear wall and, accommodated in the housing, at least one printed circuit board which stands essentially perpendicular to the rear wall.
2. Description of the Prior Art
Devices of this kind are known in many embodiments, but the requirements specified are essentially the same. In particular, the assembly of the device should be rapid and if possible should be achievable without ancillary resources; devices connected to the mains power supply should be touch-protected; and aesthetic design of the housing should be possible. Electronic devices generally also contain power semiconductors which must be cooled. The semiconductors are connected electrically to the printed circuit board and are in mechanical or thermal contact with a cooling element or a relatively large metal cooling surface.
For example, a synthetic material housing is known wherein the rear wall, the two side walls and the upper and lower wall are designed in one piece and wherein the synthetic material housing comprises a front wall which covers at the front the printed circuit board and its components which have been inserted into the housing. In order to cool certain components, cooling fins are fitted to the printed circuit board, which leads to considerable expenditure in terms of mechanical and production technology. The power semiconductors are, on one side, soldered to the printed circuit board and on the other side, mechanically held onto the cooling fins with the assistance of clips. However, this means that the cooling fins must be connected rigidly in a mechanical manner to the printed circuit board, otherwise the soldered joints of the power semiconductors and their connections may be damaged during the course of time and under adverse conditions. In the case of devices which are to be mounted on standard mounting rails, there is an additional requirement that the device should be suspended with its rear wall on a rail and the corresponding introduction of forces must be taken into account.
DE 44 22 114 A1 describes a housing pot for electronic modules, e.g. in motor vehicles. The housing pot is designed to be open at one side; it is sealed with a separate cover which is firmly connected to the printed circuit board. Heat is conducted away from the power components by means of a screw fastening of the housing pot with a cooling element, which is designed in one piece with the cover. In accordance with its specifications, this housing is attached to the housing pot via the air-duct fastening, for which reason, the choice of this housing wall as the wall which bears the printed circuit board and closes off the interior of the housing does not seem expedient. Furthermore, the realization of the connection between the cover and the printed circuit board as a fixed connection in the manner of a screw connection or similar is a disadvantage. In an electronic device publicized in EP 0 793 407 A2, the single circuit board forms one housing wall and is fitted with cooling ribs on its exterior side for the removal of heat. The printed circuit board in this case is borne mechanically by the housing, which has a synthetic material plinth with downward-protruding, flat plugs. This prior art solution is indeed suitable for small, compact devices, namely, as listed in the document, for electronic relays for plug-in assembly in motor vehicles.
By contrast, the object of the invention is to create an electronic device of the type provided above, wherein at least one printed circuit board stands essentially perpendicular to the rear wall, wherein it is possible to achieve adequate mechanical strength, and whilst taking safety requirements into account to achieve favorably priced assembly.
In accordance with the invention, this object is resolved with a device of the kind named above in that the rear wall is a part which is independent of the housing, is in thermal contract with components to be cooled and closes off the interior of the housing to the rear, and wherein the printed circuit board is connected to the rear wall in a bearing manner by holding means protruding from the rear wall and engaging with the printed circuit board.
The initial result of the measure according to the invention is that the printed circuit board is borne mechanically by the rear wall, which also has a favorable effect when mounting on mounting rails, wherein, if required, the printed circuit board can still be supported by the remainder of the housing which is subsequently fitted. One particular advantage in this case results from the fact that semiconductors arranged on the printed circuit board can now be cooled with the assistance of the rear wall or parts of the rear wall, because mechanically the rear wall forms a unit with one or more printed circuit boards which protrude perpendicularly from it.
The printed circuit board can be attached inexpensively and reliably to the rear wall if the bores of the holding means in the printed circuit board contain penetrating and clinch-connectable webs.
Assembly is rendered simpler if the bores of the holding means in the printed circuit board contain allocated alignment pins.
If the rear wall consists of an outer plate and an inner plate, wherein the inner plate comprises regions which are located at a distance from the outer plate from which the holding means protrude, a particular flexibility in cooling the components and also an improved cooling effect can be achieved because air circulates around both sides of the protruding regions. In this context, it is expedient if the inner plate is partially in contact with the outer plate and has regions essentially parallel to the outer plate and running at a distance from the outer plate. In this case, it is preferable with regard to cooling if at least the inner plate is made of metal.
With reference to cost-favorable production, it is advisable if the inner plate is riveted or Tox-clinched to the outer plate.
The rear wall may have hook elements on its outer side for suspension onto a mounting rail, which, especially in the case of a metal rear wall, proves to be a simple embodiment. In this case, the rear wall below the hook element can have at least one releasable retaining element engaging with the mounting rail.
With one simple and expedient variant of the invention, it is possible for the cooling elements to be in contact with the inner plate of the rear wall, wherein, in this case, the inner plate can be connected directly to the printed circuit board, which produces mechanical advantages in many cases, because a connection between the rear wall and the printed circuit board need not be made completely at the edge of the printed circuit board.
One variant which meets the demand for inexpensive assembly is characterized in that the components to be cooled are pressed with the assistance of U-shaped holding springs against the rear wall or its inner plate, wherein the feet of the holding springs penetrate apertures in the rear wall and their web-parts are in contact with the component.
To simplify assembly, it is preferable if the front wall, the side walls and the upper and lower wall of the housing are designed in one piece and if the housing is made of a synthetic material.
The invention and its further advantages are explained below with reference to exemplified embodiments illustrated in the attached diagram.